1. Field of the Invention
The present invention relates to an apparatus for and method of recording and/or reproducing highly-compressed image data such as HDTV (high definition television) data and low-compressed image data such as existing broadcasting data with the same data rate.
2. Description of the Related Art
Improvements of television receivers and improvements of broadcasting systems have been made in various countries by various organizations and various enterprises since the existing NTSC color television broadcasting was started in 1954 in the U.S.A. Recently, the research and the development of the HDTV system have received a practical application as a new broadcasting system that intends to obtain a high picture quality and a high definition image. In Japan, the NHK (Nippon Hoso Kyokai) has proposed a Narrow MUSE (multiple sub-Nyquist sampling encoding) system and is now carrying out a test broadcasting. Also in the U.S.A., new systems for the HDTV system broadcasting are examined under the title of ATV (Advanced Television) system.
The Journal of the Institute of Television Engineers of Japan (see Vol. 46, No. 3, pp. 276-283, 1992) has already described the aforesaid ATV system under the title of "EDTV and ATV, Trends in the U.S. and European Countries." At present, six systems are proposed as the ATV systems. Of the above-mentioned six systems, four systems relate to digital broadcasting systems in which image data is converted into a variable length code by a high-efficiency coding process and then transmitted.
A conventional system called a DigiCipher system that has been proposed by GI (General Instrument) Corporation as one of the aforesaid four systems will be described below. Since an HDTV signal has an information amount several times as large as the information amount of the existing TV signal, the broadcasting system must remove a redundancy from an original signal and transmit the HDTV signal in order to effectively utilize a broadcasting band. The DigiCipher system is a hybrid system that utilizes a DCT (discrete cosine transform) processing and a motion compensation predictive coding.
FIG. 1 of the accompanying drawings shows in block form an encoding circuit of the DigiCipher system. As shown in FIG. 1, there is provided an input terminal 90 to which digital image data a is applied. The digital image data a is supplied to a difference calculator (or subtracter) 91 which calculates a difference between the digital image data a and image data b of a preceding picture that was previously processed by the motion compensation predictive coding process. Difference data from the difference calculator 91 is supplied to a DCT circuit 92, in which it is processed in a DCT (discrete cosine transform) processing fashion. The data thus processed by the DCT circuit 92 is supplied to a quantizing circuit 93, in which it is processed by the adaptive quantization processing. The data from the quantizing circuit 93 is supplied to a variable length coding (VLC) circuit 94, in which it is processed by the variable length coding processing. Then, the data thus processed by the variable length coding circuit 94 is supplied to a buffer circuit 95, in which its bit rate is smoothed. Then, a variable length code of a constant bit rate is output from an output terminal 96. The image data b is predicted image data generated through inverse quantizing circuit 97, inverse discrete cosine transform circuit 98, adder 99, frame delay circuit 100, motion compensating circuit 104, motion vector detecting circuit 103 and switching circuits 101, 102. Motion vector detecting circuit 103 detects a motion vector from image data inputted from input terminal 90 and the last frame image outputted from frame delay circuit 100, and the predicted image data is generated by motion compensating circuit 104 based on the motion vector. Switching circuit 102 is used to switch between intra frame and inter frame coding, and switching circuit 101 is used to open and close a loop cosisting of adder 99, frame delay circuit 100 and motion compensating circuit 104. The image data is converted into a two-dimensional frequency component by the DCT processing. Man's sense of sight is sensitive to the change of a low frequency component but is relatively insensitive to the change of a high frequency component. Therefore, effectively utilizing such visual characteristics of a human being, the code amount can be reduced by the adaptive quantization processing in which different quantization bit numbers are allocated to different frequencies such as when the low frequency component of the image data processed in a DCT fashion is given a long bit length and the high frequency component thereof is given a short bit length. In the variable length coding process, a frequency component that has been processed in a two-dimensional quantization fashion is rearranged into a one-dimensional data row by a predetermined method to thereby construct a zero run length (combination of the number of consecutive zeros and the values of non-zeros) which is then processed in a two-dimensional Huffman coding fashion. In the Huffman coding process, the code amount can be considerably reduced by allocating code words of code lengths corresponding to the frequency to every zero run length.
Although the data amount of the image data can be considerably compressed as described above, a problem of error propagation occurs when image data is transmitted in the broadcasting in actual practice because the output code is the variable length code data. Therefore, the variable length coded image data is further processed in an error correction encoding fashion. In this case, the variable length coded image data is added with digital audio data and additional information and then RF-modulated by a 16 QAM (quadrature amplitude modulation) or 32 QAM system for broadcasting.
When the aforesaid system becomes a reality, the development of an apparatus that can record such image data on a recording medium becomes indispensable to such system. Under the present condition, however, in order to record the ATV image data, the image data that has been compressed and encoded by the ATV receiver must be reconverted into the original image data and the restored image data must be compressed one more time by the recording apparatus, whereafter such compressed image data must be recorded and/or reproduced by the recording and/or reproducing apparatus such as a digital VTR (video tape recorder) or the like.
Furthermore, a VTR that can record and/or reproduce both of the HDTV studio signal and the ATV signal needs two decoders and two expanding apparatuses for the broadcasting system and the recording and reproducing system. There is then the problem that the circuit scale of such VTR is enlarged.